Add trace scenario for partial AppendEntries overlap

[Copilot]

Closes #7635.

Trace validation did not model a valid Raft behavior where an AppendEntries message starts inside a follower’s existing log prefix, overlaps matching entries, and extends past the follower’s end.

• Raft scenario

  • Adds partial_append_entries to reproduce this shape explicitly.
  • The generated trace includes receives where:

    prev_idx < follower_last_idx < idx
    

• TLA trace model

  • Adds PartialAppendEntriesRequest to accept matching overlap plus appended suffix.
  • Wires the new case into AcceptAppendEntriesRequest.
  • Documents why conflict handling still falls through to the existing post-truncation cases.